Method of carburizing tungsten filaments



Oct. 18, 1955 P. D. WILLIAMS METHOD OF CARBURIZING TUNGSTEN FILAMENTS Original Filed Feb. 27, 1950 I N V EN TOR. Pau/ L7 W/H/ams ATTORNEY United States Patent METHOD OF (IARBURIZING TUNGSTEN FILAMENTS Paul D...Williams,:-Palo- Alto,-:-.Calif=,=assignor to Eitel- McCuIIough,.Inc.,-San-Bruno,' Calif., a corporation of California Original:applicafi0i1 Febrllary 27;1950; Serial No.

' '3 Claims. (Cl.'148'13.1)

-..-'Ihis is a division of-my copendingapplication Serial .No. 146,507,.filed February 27, 195-0, now vUAS. tPatent My invention relatesto-filamentary.eathodes foruelec- .tron tubes, and .more particularly .to;.-improvements in thoriated tungsten type filaments.

. 'Ihoriated tungsten .wirecomprising-tungsten containing thorium oxide :(thoria) is :widely used in; making filaments for. electrondischargedevices such as radio transmitting tubes. The practice is to form.:such filawments of solid-.thoriated tungsten wire, shaped as hair- :pinsorother desired shapes, which filaments are. mounted on-a-suitablerstem structure. In ordento; improve the electron emission properties rof thev filament thew :thoriated tungsten wire is; carburized to convert the outer por- .tions vof the .wireto-tungsten" carbide, thexpractiee being to heat thesmountedfilament in anatmosphererontainmercial and military applications, particularly .when the ntube 1 is subjected to shock andvibration.

.-.The broad-object of my-invention is to improve the strength of .thoriated tungsten-filaments.

- Another -objeet is to achieve the improved strength without impairing the electron emission propertiesof the filament.

A further object is to providea 'filamentmaking material which may be readily fabricated in the initial stage, and ;which is;;easily handlede and processed during tube manufacture.

The invention possesses other objects and features of set-'forth inthe following-description of my-' invention. It is to be understoodthat I do not' limit myself to this disclosure of species of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is an elevational view of a filament structure embodying my improved filaments.

Figures 2 and 3 are enlarged cross-sectional views of a filament before and after carburization; and

Figures 4 and 5 are side views of the same.

Figures 6 and 7 are cross-sectional views of modified type of filament before and after carburization; and

Figures 8 and 9 are side views of the same.

In terms of broad inclusion, my improved filament for electron tubes comprises a bundle of twisted strands of thoriated tungsten Wire. In its preferred form the bundle is made up of several twisted wire ropes, each rope comprising twisted strands of the thoriated tungsten wire. Preferably only certain portions of the stranded filament are carburized to form a carburized zone running helical ice 2 ly of. the filament. To facilitate! carhurization. of the filament, a carbonrbea iing. thread is preferably-intertwistedwith the-wire, which-thread is preferably .wound helically of. the filamentaaxis. Thei.-filament -material thus formed issuitably. mounted on-a supporting'stem,

-after..which. the-filamenttis heated to...disintegrate the thread and carburize the wire.

.In greater detaihand referringfirst rfqFigure l -.of the drawing, my improved-filament forelectron tubeszisillustrated asa. filamentary cathodetstructurescornprising a plurality of parallel filaments 10...- Thenstructure shown .has. a glass stem llcarryingapair.ofnietallicleads 12 anda centralstandard 13 sealed to, a. press 14 of the stem; :Flexible extensions 16 on theleads serveetomake connectionshwith the base ofv the :tube. cup shaped v metal'shield 17- fastenedtozstandard 13vand'embracing thezpress 14 serves-topretect the-.glasssstem.

-. port 18 on one of theleads 1-2, .andthe lower. ends of In the filament rstructureuillustrated, the:;filaments 10 aresmaintainedsunder.tension. -The lower-.ends of certain of the filaments: are-vvelded'tow a fixed arcuate supthe. remaining filament-s. are -welded .tosa second fixed arcuatei support 18 onttheeotherxlead.12. All of the -...filaments are welded-rat theg-upper endto awcircular-sup- 2 port 19's1idable on therstandard. 13;, By.this...ar.r;angement the filamentheating current: passes up throughhalf -of'thefilamentsand-down :through the other half-..:.The

, :filaments: are..tensioned by= a-.;coil spring 22s-encircling a standard 13. and interposed betweenea. stop .23- and acompressionmsleeve 24:extending upwardly and-bearing against a cup 26runder. the slidablesupport 19.

The above described,astem construction is merely for purposes ofiillustrationgitabeingsunderstoodnthat other suitable-filament.supportingstructures may beaemployed.

- thoriated gtungsten-awire- In. the form shown in Figures .--Tlreaindividualfilaments 10-embodying my improvementseach comprise -:a-..bundle of ..twisted-.strands of 2 to 5- the filament'is. made up of-ra uniform-circular bundle of twistedsstrands 27.. .--The..strands 27: are of an ordinary commercialigrade of thoriated tungsten wire, the'size andnumber ofvzstrands dependingsupon the diameter of filament desired. .T116-dl'8.l11l181' of the individual-strands is preferably quite smallv so that arelatively large number ofustrandsareprovidedsin the :bundle, the

: multiplicity ofasmalltstrands being-partiallyresponsible for the improved strength compared to the usual-filament of-solidcrossr-sectionnnFonexample l-aprefer to use 2l..-strands ofeOOZinchndiameter wire-tonproduce a. filaadvantage, some of--which;--with the foregoing; will be --have-been used ibutewires of..sueh---small.diameter are "harder toe-handle ina-..tvvistingmachine. .sThe strands a: may betwistedg together in; any-"suitable type of twisting machine, a suflicient number of twists being provided to hold the strands together.

As above mentioned, the stranded character of the filament is partially responsible for the improved strength and pliability of the filament. Another very important factor, in combination with stranded feature, is the distribution of carbon in the final carburized product. The stranded filament is only partially carburized, preferably to provide a carburized zone running helically of the filament, so that a quantity of uncarburized strands are preserved to maintain the strength of the filament. Such distribution of carbon is conveniently accomplished by intertwisting with the strands 27 a carbon bearing thread 28. This may be an ordinary white cotton thread and may be impregnated with aquadag, if desired, to increase the carbon content. The thread is preferably incorporated so that it lies as a helix along the filament, ad-

' section.

jacent the outer strands of wire in the bundle, as shown in Figures 2 and 4. Y

The wire so fabricated and including the thread may be produced in long lengths by the wire manufacturer and supplied on the spool to the tube industry, in the same manner that solid thoriated tungsten filament making wire is now supplied to the trade. At the tube plant the stranded filament material is cut to length and the filaments are mounted by welding or otherwise to the filament supporting in the usual manner. During this mounting stage the stranded filaments have an advantage because they are more flexible and therefore more easily handled than filaments of solid cross section.

After mounting, the improved'filaments are carburized by the simple expedient of heating the filaments in an oxygen free atmosphere, such as in hydrogen or in a vacuum, to a temperature sufiicient to disintegrate the thread and carburize the wire. Heating the filaments to a temperature of about 2100" C. is adequate for this purpose. The thoriated tungsten strands adjacent the thread are thus carburized in a zone 29 running helically of the filament as indicated in Figures 3 and 5.

In order'to obtain maximum strength I prefer to use a form of stranded filament shown in Figures 6 to 9 wherein the bundle is made up of several twisted ropes 31, each rope comprising twisted strands 32 of the thoriated tungsten wire. For example, I have had good results using three twisted ropes, each rope comprising seven twisted strands of .002 inch diameter wire, to produce a filament having electron emission properties similar to a .010 inch diameter filament of solid cross As seen in Figure 8, the strands 32 of a given rope preferably have tighter twists than the twists between the ropes 31. This provides an interlocking action where the strands bind together between the ropes.

In this case the carburizing thread 33 is preferably intertwisted with the ropes 31 so that the thread follows the twist of the wire ropes as shown in Figures 6 and 8.

After heating, a carburized zone 34 is provided running helically of the filament as indicated in Figures 7 and 9. It will be noted, because the individual strands have tighter twists than the ropes themselves, that the carburized zone runs crosswise of the separate wire strands. In other words; the carburized zone 'runs across the strands rather than following individual strands as in the case first described. Thus, a given strand is carburized in sections only, and, since the remaining sections of the strand are interlocked between the ropes, an additional measure of strength is obtained in the filament,

I have found that filaments made in accordance with my invention have good electron emission efiiciency and filament life characteristics. I havealso made extensive shock tests to compare'the improved filaments with those commonly used in the art. These comparative tests were made with electron tubes having filament stem structures similar to that shown in Figure 1, the tubes being substantially the same except that in one case the filaments were of the carburized stranded type shown in Figures 7 and 9, and in the other case were ordinary carburized solid wire filaments. A standard impact machine for electron tubes was employed in making the shock tests. The results showed that the ordinary filaments broke at an average of about G. shock acceleration, while the improved filaments broke at an average of over 400 G., constituting an improvement of better than 300% in shock resistance.

Instead of employing a thread to carburize the filament, my improved stranded type filament may be carburized in the ordinary manner by heating it in a hydrocarbon atmosphere. For example, the stem shown in Figure 1, with the stranded filaments mounted in place, may be positioned in a belljar containing a hydrocarbon atmosphere, such as benzene, and the filaments heated sufficiently to absorb carbon from the atmosphere. In this case the outer strands of each filament are uniformly carburized, leaving the strands in the core portions of the filament uncarburized. This type of carburized stranded filament does not have the maximum strength, but is still superior to an ordinary filament of solid cross section carburized in the same manner. Compared to ordinary filaments breaking at an average of about 100 G. shock acceleration, the last mentioned stranded filaments broke at an average of about 200 G., showing an improvement of about 100% in shock resistance.

I claim: 7 I

1. The method of making a filamentary cathode, which comprises mounting a filament containing a thread of carbon bearing material and thoriated tungsten wire, said thread engaging a selected portion only of the surface of the wire, and then heating the filament to a temperature of about 2100 C. to disintegrate the thread and integrate the thread and carburize the selected'portion of the Wire.

3. The method of makinga filamentary cathode, which comprises mounting a filament containing carbon bearing thread helically wound with strands of thoriated tungsten wire, said thread engaging a selected portion only of the surface of the wire, and then heating the filament to a temperature of about 2100" C. todisintegrate the thread 'and'carburize the selected portion of thewire.

References Cited in the file of this patent UNITED STATES PATENTS 376,194 Harvey Jan. 10, 1888 2,397,553 Chevigny Apr. 2, 1946 2,495,167 Horstman et a1. Ian. 17, 1950 um-l 

1. THE METHOD OF MAKING A FILAMENTARY CATHODE, WHICH COMPRISES MOUNTING A FILAMENT CONTAINING A THREAD OF CARBON BEARING MATERIAL AND THORIATED TUNGSTEN WIRE, SAID THREAD ENGAGING A SELECTED PORTION ONLY OF THE SURFACE OF THE WIRE, AND THEN HEATING THE FILAMENT TO A TEMPERATURE OF ABOUT 2100* C. TO DISINTEGRATE THE THREAD AND CARBURIZE THE SELECTED PORTION OF THE WIRE. 